It is well-known that a front panel on an occupant side of an airbag and a rear panel on the opposite side thereto are disconnectably connected by means of a plurality of circular line-shaped connecting portions that are approximately concentrically extended in a peripheral direction of the airbag at halfway portions disposed halfway between respective center portion and peripheral edge portion thereof (for example, Japanese Unexamined Patent Application Publication No. 8-156730, etc). This connection-releasable line-shaped connecting portion is constructed to release a connection of the front panel and the rear panel by tearing when internal pressure of the airbag is brought to a predetermined pressure or more.
In a case that a gas is supplied into the thus constructed airbag from a gas-generator and the airbag is expanded, since the connection of the front panel and the rear panel by the line-shaped connecting portion is not released until the internal pressure of the airbag is brought to the predetermined pressure or more, the expanding operation of a portion on the outer periphery side relative to the line-shaped connecting portion in the airbag is limited. Accordingly, the pressure in the airbag is rapidly raised by means of the gas from the gas-generator.
When the internal pressure of the airbag is brought to the predetermined pressure or more, each of the line-shaped connecting portions is torn and the connection of the front panel and the rear panel is released. As a result, the airbag is brought to a state to be able to expand up to an outermost peripheral portion thereof. At this moment, the internal pressure of the airbag is maintained to be high pressure during the time from an initial stage of the expanding operation of the airbag to a completion of the expanding operation thereof by means of tearing the line-shaped connecting portion from the line-shaped connecting portion on the innermost periphery side to the line-shaped connecting portion on the outermost periphery side in a sequential manner (stepwise manner).
In order to start a tearing operation of the line-shaped connecting portion from a predetermined position, it is described in FIG. 2 of the aforementioned Japanese Unexamined Patent Application Publication No. 8-156730 that a communicating portion (appellation is not shown in the Japanese Unexamined Patent Application Publication No. 8-156730) that allows an inner periphery side and an outer periphery side relative to a line-shaped connecting portion to communicate with each other, while a connection of the front panel and the rear panel is partially interrupted, is provided at a halfway portion in an extending direction of the line-shaped connecting portion (called as a stitching portion in the Japanese Unexamined Patent Application Publication No. 8-156730). In the thus constructed case, when the internal pressure of the airbag is brought to a predetermined pressure or more, a stress is concentrated to an end portion of the line-shaped connecting portion facing the communicating portion, and the tearing operation of the line-shaped connecting portion is started from the end portion.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 8-156730
In FIG. 2 of the aforementioned Japanese Unexamined Patent Application Publication No. 8-156730, any of the communicating portion of the line-shaped connecting portion on the innermost periphery side, the communicating portion of the line-shaped connecting portion on the outermost periphery side, and the communicating portion of the line-shaped connecting portion in between these are disposed in a positional relationship of overlapping with each other in a peripheral direction of the airbag (equal phase relationship).
As described above, when the communicating portion of the line-shaped connecting portion on the inner periphery side and the communicating portion of the line-shaped connecting portion on the outer periphery side are disposed at a position overlapping with each other in the peripheral direction of the airbag, or a position relatively close to each other, the internal pressure of the airbag is brought to be instantly applied to the communicating portion of the line-shaped connecting portion on the outer periphery side after the tearing operation of the line-shaped connecting portion on the inner periphery side is started. Therefore, there is a possibility that the line-shaped connecting portion on the outer periphery side also starts tearing operation before finishing the tearing operation of the entire line-shaped connecting portion on the inner periphery side.
In this case, the tearing operation may locally proceed up to the line-shaped connecting portion on the outermost periphery side in the peripheral direction of the airbag, or a portion in the line-shaped connecting portion, which is spaced apart from the communicating portion, may partially remain without being torn. Thereby, the airbag becomes difficult to be evenly expanded in the peripheral direction thereof.
Further, in FIG. 2 of the aforementioned Japanese Unexamined Patent Application Publication No. 8-156730, since a venthole is provided in the vicinity of the communicating portion of the line-shaped connecting portion on the outermost periphery side, there is a possibility that the gas in the airbag is discharged outside the airbag from the venthole, while passing through the communicating portion in a shortcut manner before finishing the tearing operation up to the line-shaped connecting portion on the outermost periphery side. In this case, there is also a possibility that an elevation of the internal pressure of the airbag and the tearing operation of the line-shaped connecting portion along with that are delayed.